The present invention relates in particular but not exclusively to injection and particularly to injection of a hydrocarbon charge into a fluidized-bed catalytic cracking reactor.
In fluidized-bed catalytic cracking processes known in the field as FCC processes, the hydrocarbon charge is injected into the reactor in the form of a column, in which the catalyst is held in suspension and moves either in an essentially upward flow (the reactor is then called a xe2x80x9criserxe2x80x9d) or in an essentially downward flow (the reactor is then called a xe2x80x9cdropperxe2x80x9d).
For additional clarity, reference will be made hereinbelow to the case of risers, but the transposition to droppers will be evident to the individual skilled in the art, and the charge injectors subject of the present invention apply of course to both types of reactors.
In a riser, the regenerated catalyst is introduced at the base of the reactor at the same time as a carrier gas, in a hot, fluidized state (between 650 and 850xc2x0 C.), below the hydrocarbon charge injection zone. This charge is introduced into the reactor in the essentially liquid state and at a temperature generally between 80 and 350xc2x0 C.
Between the charge injection zone and the top of the reactor, the catalyst yields some of its energy to the charge, which is then vaporized and cracked into light hydrocarbons. This has the effect of rapidly increasing the volume of gases which carry the catalyst in accelerated fashion up to the top of the reactor, where it is separated from the hydrocarbons. The mixture of hydrocarbons and catalyst seeds reaches an equilibrium temperature at the riser outlet which is normally between 470 and 600xc2x0 C.
During these operations, a small part of the charge (generally between 3 and 12 wt. %) forms a solid hydrocarbon deposit or xe2x80x9ccokexe2x80x9d on the catalyst particles which reduces the catalytic activity of the catalyst and limits the conversion of the charge into products from which value can be extracted. Hence it is necessary to regenerate the catalyst by burning off this coke deposit before reintroducing it into the reactor for a further cracking cycle.
The size of the coke deposit on the catalyst is generally in proportion to the weight of the injected charge.
Moreover, recent developments in catalytic cracking have shown that important factors in the cracking reaction are the speed and uniformity with which the charge is placed in contact with the catalyst seeds, and hence the quality of atomization and vaporization of this charge when it is injected into the reaction zone.
A number of systems for injecting the hydrocarbon charge into the reaction zone of the catalytic cracking reactor have been described in the prior art.
U.S. Pat. No. 4,097,243 describes for example a charge injection system including a series of tubes disposed at the end of a cone allowing the charge to be distributed over all the moving catalyst seeds. Such a system however has the disadvantage of triggering contact of part of the liquid charge with the reactor walls, leading to excessive, harmful formation of coke.
U.S. Pat. No. 3,812,029 describes a particular injector for obtaining fine particles of charge; however, this injector does not allow good distribution of the droplets, hence poor vaporization of the hydrocarbons and excessive coking. Moreover, when heavy charges are used, this injector often and very rapidly becomes clogged.
Patent application EP-A-220,349 describes a particular injector having a spiral for obtaining droplets with a mean diameter of less than approximately (35xc3x9710xe2x88x925 meter) 350 microns while at least partially avoiding contact of the charge with the walls of the reaction zone. However, formation of vortices of charge droplets near these injectors interferes with the flow of the catalyst seeds, causing an increase in the backmixing phenomenon which is detrimental to proper operation of the process.
The usefulness of injecting the charge to be processed into the reactor at high speed and in the form of very fine droplets is well known in the prior art (see U.S. Pat. Nos. 2,891,000 and 2,994,659).
The goal of the present invention is to overcome the drawbacks of the prior art hydrocarbon charge injection process and to obtain, particularly in the case of cracking heavy charges, optimum contact between the hydrocarbons and the catalyst seeds due to homogeneous, instant atomization in the form of fine droplets in the injection zone.
More specifically, the present invention allows smaller droplets to be created than those formed in a single-neck venturi injector with an equivalent passage cross section at the level of the neck and operating under the same conditions.
In other words, the present invention improves the performance of a venturi-type injector by increasing the efficiency of contact between the gas phase and the liquid phase in the injector.
The venturi injector is particularly well suited for injection of hydrocarbons in a catalytic cracking process. It comprises, as a minimum, a mixing chamber into which the hydrocarbon charge and the atomization gas are introduced at low speed, a converging section in which the liquid and gas phases are mixed in a co-current, and a diverging section in which the gas and liquid flow homogeneously, the liquid being atomized into fine droplets. This injector atomizes the hydrocarbons into fine droplets, using a low percentage of vapor.
Moreover, the present invention allows less coke to be formed.
The consumption of vapor, for a given droplet size, is smaller. The associated reduction in operating costs is highly appreciated by users.
Another advantage of the present invention relates to the compactness of the equipment; the comparative data cited below will allow this advantage to be better appreciated.
Thus, the present invention relates to a device for injecting a hydrocarbon charge into a catalytic cracking reactor having in particular an envelope, a main gas feed tube, and an inlet for an auxiliary gaseous fluid.
According to the invention, the charge feed tube is divided, inside the envelope, into at least two secondary lines designed to distribute said charge uniformly, each secondary line terminates upstream of a venturi-type element, and the auxiliary gas fluid is mixed with the charge in a mixing chamber located at the inlet to said venturi.
In particular, the venturi-type elements are disposed parallel to each other inside the envelope.
According to one embodiment of the invention, the inlet for the auxiliary fluid is disposed parallel to at least one of said secondary lines.
The at least one secondary line can be fitted on its inside with a restriction in passage cross section. According to one embodiment of the invention, the venturi-type element has an extension at its converging section.
Advantageously, the envelope can have a mouthpiece having an opening, located downstream of said venturi and designed to direct the mixture of charge and auxiliary fluid to a reaction zone outside the envelope.
The mouthpiece can be arranged to create a flat jet.
More specifically, the walls of the opening of the mouthpiece define an angle xcex1 of between approximately 30 and 75xc2x0.
According to one feature of the invention, the venturi-type elements have a converging section with an angle of between 10 and 45xc2x0 and a diverging section with an angle of between 2 and 14xc2x0.
According to a first embodiment of the devices according to the invention, each secondary line has at least one orifice which terminates substantially coaxially upstream of the venturi.
According to a second embodiment of the device according to the invention, each secondary line can be blocked at its end and have a plurality of orifices or slots disposed at its periphery and designed to carry the charge transversely to the flow of the secondary fluid. These orifices or slots terminate downstream of the venturi and deliver the charge preferably substantially perpendicularly to the secondary fluid flow.
The device according to the invention can be applied to catalytic cracking of heavy hydrocarbon charges.